A.J. Sobey

Monte Carlo reliability analysis of tophat stiffened composite plate structures under out of plane loading

Composite materials are often utilised for their high strength to weight ratio, excellent corrosion resistance, etc. but are also characterised by variabilities and uncertainties in their mechanical properties owing to the material make-up, process and fabrication techniques. It is essential that modelling techniques continue to be developed to take account of these variabilities and uncertainties and as more complicated structures are developed it is important to have rapid assessment methods to determine the reliability of these structures. Grillage analysis methods have been previously used for assessment of tophat stiffened composite structures using simple failure criteria. As new criteria are introduced, such as by the World Wide Failure Exercise, the response of more complex topologies must be introduced. This paper therefore assesses the reliability of composite grillages using Navier grillage method incorporating up to date failure criteria. An example, taken from boatbuilding, is used to show the results of using these more complex assessment methods showing that it is of high importance to use the correct assessment criteria.

Optimisation Approaches to Design Synthesis of Marine Composite Structures

Abstract This paper develops a method of structural optimisation for stiffened FRP panels allowing optimisation between cost and mass. The approach uses genetic algorithms for optimisation combined with elastic stress modified grillage theory for stiffener structural analysis and third-order shear deformation theory for the plate structural analysis. For top-hat stiffened panels, optimum designs following first principles and classification society rules are compared, showing considerable potential for cost savings by using a first principle approach.

Determination of critical factors for the strength assessment of damaged steel ship structures

A significant number of damage events continue to occur to ocean going vessels, many of which remain afloat in need of assistance to evaluate remedial actions to minimise the risk of further damage and conditions for onward transit for a repair facility. Therefore, it is vital that the post damage situation is rapidly understood to reduce damage propagation occurring, reducing the cost of repair and allowing more vessels to be recovered safely, ensuring the safety of personnel onboard. After an incident, it is often difficult to determine and model the precise damage scenario due to the inability to survey the area. Each scenario will have variability in geometrical and material properties, which will affect the residual strength of the structure. Variations in these aspects are not accounted for in methods currently utilised in damage response scenarios. Therefore, to be able to more accurately analyse the damage and provide better guidance to the crew in real time, it is important that this variability can be analysed, allowing an understanding of worst and best case scenarios, the probabilities of these occurring and their affect on the structural strength. Finite element analysis has been used to model damage scenarios due to the high level of accuracy that can be achieved. This paper demonstrates the implications of damage aperture on the limit state of stiffened steel panels, investigating the residual strength of the damaged structures and their sensitivity to variations in damage event, geometric and material properties. The results are then compared with the Smith and Dow progressive collapse method, [1,2], with conclusions being drawn about the use of this method in damage situations. Further to this, the effects of variability in the ship are investigated for damage scenarios, showing that these influence the ultimate strength of the structure to a larger extent in the failure of damaged plates than intact scenarios; however, lack of knowledge relating to the area of the damage could overshadow other potential variability within these scenarios.

Optimisation of FRP structures for Marine Vessel Design and Production

Structural integrity is an important factor in design and production of any vessel. The design and production of the structure will permeate throughout the other subsystems of a vessel. These interactions require that the structure must be light, hydrodynamic and easy to build allowing an efficient and low cost design to be constructed. During the design process engineers use iterative design methods to improve optimum designs. The ability to have effective reliability data will allow this iteration to be a more quantitative process. The addition of these methods to optimisation techniques will allow a powerful tool that optimises design while producing data for other designers to determine possible changes that could take place. This paper looks at using reliability methods to increase the power of design for production within concurrent engineering and structural optimisation.© 2009 ASME

Behaviour of Multi-Level Selection Genetic Algorithm (MLSGA) using different individual-level selection mechanisms

Abstract The Multi-Level Selection Genetic Algorithm (MLSGA) is shown to increase the performance of a simple Genetic Algorithm. It is unique among evolutionary algorithms as its sub-populations use separate selection and reproduction mechanisms to generate offspring sub-populations, called collectives in this approach, to increase the selection pressure, and uses a split in the fitness function to maintain the diversity of the search. Currently how these novel mechanisms interact with different reproduction mechanisms, except for the one originally tested at the individual level is not known. This paper therefore creates three different variants of MLSGA and explores their behaviour, to see if the diversity and selection pressure benefits are retained with more complex individual selection mechanisms. These hybrid methods are tested using the CEC′09 competition, as it is the widest current benchmark of bi-objective problems, which is updated to reflect the current state-of-the-art. Guidance is given on the new mechanisms that are required to link MLSGA with the different individual level mechanisms and the hyperparameter tuning which results in optimal performance. The results show that the hybrid approach increases the performance of the proposed algorithms across all the problems except for MOEA/D on unconstrained problems. This shows the generality of the mechanisms across a range of Genetic Algorithms, which leads to a performance increase from the MLSGA collective level mechanism and split in the fitness function. It is shown that the collective level mechanism changes the behaviour from the methods selected at the individual level, promoting diversity first instead of convergence, and focuses the search on different regions, making it a particularly strong choice for problems with discontinuous Pareto fronts. This results in the best general solver for the updated bi-objective CEC′09 problem sets.

Reliability of Composite Marine Structures

Traditional engineering design takes a deterministic view of the world. This type of analysis has provided a safe method for designing structures for many years. However, reliability analyses have been developed to account for the stochastic loads encountered in service and the variation in geometries and material properties seen after production. There are a range of techniques available for this analysis each with their own advantages and disadvantages. This chapter explores methods for composite analysis using simplified methods and simulation techniques.

Investigating the transient response of hybrid composite materials reinforced with flax and glass fibres

Considerable attention has been devoted to the integration of natural-derived composites into the composite market. In this regard flax fibre can be considered as the most promising natural reinforcement among other natural fibres due to its high specific mechanical properties although these properties fall behind the mechanical properties of conventional fibres. Notwithstanding the downside of flax fibres against conventional fibres in terms of mechanical properties, their inherent viscoelastic characteristics bring an advantage of high vibration damping property. Evaluating the behaviour of flax fibre composites, it is also required to have an understanding of such composite material’s response to dynamic loads as structures are exposed not only to static loads but also short duration loads. This paper presents an experimental methodology for investigating the transient response of flax and glass fibre reinforced composites, as well as hybrid of the two, to impulsive loads.

Re-inspiring the genetic algorithm with multi-level selection theory: multi-level selection genetic algorithm

Genetic algorithms are integral to a range of applications. They utilise Darwins theory of evolution to find optimal solutions in large complex spaces such as engineering, to visualise the design space, artificial intelligence, for pattern classification, and financial modelling, improving predictions. Since the original genetic algorithm was developed, new theories have been proposed which are believed to be integral to the evolution of biological systems. However, genetic algorithm development has focused on mathematical or computational methods as the basis for improvements to the mechanisms, moving it away from its original evolutionary inspiration. There is a possibility that the new evolutionary mechanisms are vital to explain how biological systems developed but they are not being incorporated into the genetic algorithm; it is proposed that their inclusion may provide improved performance or interesting feedback to evolutionary theory. Multi-level selection is one example of an evolutionary theory that has not been successfully implemented into the genetic algorithm and these mechanisms are explored in this paper. The resulting multi-level selection genetic algorithm (MLSGA) is unique in that it has different reproduction mechanisms at each level and splits the fitness function between these mechanisms. There are two variants of this theory and these are compared with each other alongside a unified approach. This paper documents the behaviour of the two variants, which show a difference in behaviour especially in terms of the diversity of the population found between each generation. The multi-level selection 1 variant moves rapidly towards the optimal front but with a low diversity amongst its children. The multi-level selection 2 variant shows a slightly slower evolution speed but with a greater diversity of children. The unified selection exhibits a mixed behaviour between the original variants. The different performance of these variants can be utilised to provide specific solvers for different problem types when using the MLSGA methodology.

Experimental investigation into factors affecting the transient flow of fluid through an orifice in realistic conditions

During operations, damage can occur with a resulting ingress or egress of fluid. The incoming water affects the reserve buoyancy and it can also change stability and hull girder loading. During a flooding event it is vital that the flow through the damaged orifice and the movement of floodwater around the structure can be predicted quickly to avoid further damage and ensure environmental safety. The empirical measure coefficient of discharge is used as a simplified method to quantify the flooding rate. In many internal flow applications the coefficient of discharge is estimated to be 0.6 but recent research shows that it can vary considerably when applied to transient flooding flows. This paper uses an experimental setup to investigate how changes to the orifice edges and position within the structure affect the flow. It is then used to investigate the coefficient in a more realistic scenario, a static compartment in waves.

The Application of Reliability Based Optimization of Tophat Stiffened Composite Panels Subject to Bi-Directional Buckling Loads

Stiffened composite panels are used within many applications, from aerospace to marine applications. Stiffened panels are utilized for their high strength to weight ratio and flexibility of layups while counteracting the low stiffness exhibited by composites. Complications arise when attempting to utilize the full variability of layups in conjunction with reliability constraints creating a complex design problem when constrained by both buckling and material strength. To aid the process of optimizing the design of composite structures and layups, while ensuring a low mass, this paper presents a bi-level optimization scheme for minimization of the weight of tophat stiffened composite panels with probabilistic deflection constraints. To improve the computational efficiency, an energy based grillage method is formulated and applied for the investigation of buckling problems under bi-directional in-plane loads. The method is validated by comparing the results obtained from FE model calculations. The variables that have a large impact on the structural safety have been identified by both safety index and COV based reliability analysis. A parametric study of plate dimensions and loading ratios is conducted to investigate the coupling effects on critical buckling load. The method presented in this paper, makes it possible for engineers to improve their designs, at an early stage, with an integrated consideration between product performance and design parameters.